In recent years, in solid-state image pickup units such as CCDs (Charge Coupled Devices) and CMOS (Complementary Metal Oxide Semiconductor) image sensors, the number of photons entering a pixel is reduced with reduction in pixel size, and reduction in sensitivity is concerned accordingly. Therefore, a so-called laminated solid-state image pickup unit is adopted as a solid-state image pickup unit that makes it possible to achieve an improvement in sensitivity. In the laminated solid-state image pickup unit, an aperture ratio of a photoelectric conversion region is allowed to be increased by forming a photoelectric conversion layer above a semiconductor substrate made of silicon or the like.
On the other hand, a longitudinal spectral solid-state image pickup unit in which photodiodes of respective colors R, G, and B are longitudinally laminated has been proposed, and in this configuration, respective color signals of R, G, and B are allowed to be obtained without using a color filter. Therefore, light loss caused by the color filter is eliminated, and almost entire entered light is allowed to be used for photoelectric conversion, thereby improving image quality. In particular, a configuration in which a photoelectric conversion layer different from silicon is formed above a silicon substrate is attracting attention as this longitudinal spectral solid-state image pickup unit. This configuration makes it possible to improve color separation characteristics, compared to a case where all of photodiodes of R, G, and B are formed in a silicon substrate.
However, a typical buried photodiode configuration used when photoelectric conversion and electric charge storage are performed in the silicon substrate is not applicable to the above-described laminated solid-state image pickup unit. Therefore, a contact section configured to electrically connect the inside and the outside of the silicon substrate to each other is provided; however, this contact section causes occurrence of a dark current.
To suppress such occurrence of the dark current, there has been proposed a technique of storing an electric charge generated by photoelectric conversion in not an electric charge storage layer (an impurity diffusion layer) in a silicon substrate but a gate electrode of amplification transistor (for example, refer to PTLs 1 and 2).